Patent GB-693600-A (GB)
Title: Improvements in or relating to apparatus for measuring the magnitude of a condition e
IPC: G01F23/26
Filed: 1951-05-21
Abstract: 693,600. Electric indicating systems. LIQUIDOMETER CORPORATION. May 21, 1951 [June 8, 1950]. No. 11798/51. Class 40 (i). [Also in Group XXXVI] Apparatus for indicating a variable, e.g. the level or volume of liquid in a tank, independently of a second auxiliary variable, e.g. the dielectric constant of the liquid, comprises a first variable capacitor responsive to both variables, e.g. to the liquid level separating its plates; a second variable capacitor responsive to the second variable, e.g. by immersion in the liquid; third and fourth fixed capacitors, and a source of two equal antiphased alternating potentials; the first and third capacitors being connected respectively between the first and second potential sources and a fixed point through invariable resistive circuits and the second and fourth capacitors being connected respectively between the potential sources and the same fixed point through adjustable resistive circuits, so that the product of the second potential and the third capacitance equals the product of the first potential and the first capacitance when the first and second capacitors are unaffected by the variable to be indicated; and a voltage sensitive circuit connected to the common point operating to control a motor rebalancing the adjustable resistive circuits to produce a voltage null at the common point and simultaneously operating an indicating device for the magnitude of the variable, e.g. the tank level or volume. A measuring condenser 6 (Fig. 1) comprising concentric insulated conductive cylinders 8, 12 is immersed in liquid 4 in a tank 2, which has a well 32 containing a parallel plate condenser 24 compensating for variations in the liquid dielectric constant. In the measuring circuit (Fig. 2) the condenser 6 is connected between the upper end of the secondary of a supply transformer 42 and the output terminal 54, while the compensating condenser 24 is connected between this point and the slider of a potentiometer 62 connected between earth and the lower end of the transformer secondary. A fixed condenser 58, is connected between the output terminal 54 and the slider of a potentiometer 52 (ganged to and identical with potentiometer 62), between the upper end of the transformer secondary and earth; and a fixed condenser 66 is. connected between the lower end of the transformer secondary and the output terminal, the voltage developed on which operates an amplifier 76 energizing one winding of a bi-phase motor 68 which readjusts the ganged sliders of potentiometers 52, 62 to a balance point at which the output voltage is reduced to zero, and also drives a level or volume indicator 96. The four condensers are of equal value Co when the tank is empty of liquid, so that if K is the di-electric constant thereof and X the fractional depth of liquid in the tank, the capacity Cm of the measuring condenser = Co[1+X(K-1)] and that of the compensating condenser = KCo. Since condensers 6, 66 are connected between terminal 54 and sources of equal anti-phased voltages e, they are equivalent to a condenser of capacity C 1 =CoX(K-1) substituted for condenser 6, while condensers 24, 58 are similarly equivalent to a condenser of capacity C 2 =Co(K-1) substituted for condenser 24. If m is the potentiometer proportionality constant when the voltage at terminal 54 is reduced to zero, e CoX(K-1)-m e Co(K-1)=0 so that the liquid depth (or volume) is proportional to the balanced potentiometer setting for zero output, independently of the liquid di-electric constant. In Fig. 3, identical ganged calibration potentiometers 108, 112 are energized across similar portions of the secondary of transformer 102, and their slider voltages energized the ganged identical rebalancing potentiometers 52, 62 which are adjustable as before by motor 68 (also driving indicator 96) to restore zero voltage at output terminal 54. Fixed condenser 58 and compensating condenser 24 are connected between the respective balance potentiometer sliders and the output terminal, while the measuring condenser 6 and the fixed condenser 66 are connected between the output terminal and the ends of the transformer secondary through equal resistances 122, 124. The condenser-resistance junctions are interconnected by the resistance chain 146 to 154 of a densitometer of the kind described in Specification 684,348 wherein floats of graduated densities sink or float to short or open circuit successive resistances of the chain, according to the liquid density. As before the motor readjusts the balancing potentiometers for zero output voltage at terminal 54, and at balance the pairs of condensers 6, 66; 24, 58 are as before equivalent to single condensers of capacity CoX(K-1) and C o (K-1) replacing condensers 6 and 24 respectively. The liquid density is given by R/R+2r=PD where R is the densitometer resistance, r the resistances 122, 124, and P a proportionality constant, while the anti-phase voltages on the sliders of the balance potentiometers are equal to Œ s m e; s, m being proportionality constants depending on the settings of potentiometers 108, 112 and 52, 62. Then at balance. m s e (K-1)Co - e.R/R+2r Co(K-1) X = 0 which reduces to XD=sm/P. If the measuring condenser plates are shaped so that X is proportional to the volume of liquid in the tank, the weight of liquid is proportional to the indicated setting m of the balance potentiometers. In a modification Fig. 4 (not shown) the measuring circuit is re-arranged so that the motor re-adjusts the slider of a single balance potentiometer in a symmetrical resistance chain across the supply transformer secondary; and a variable calibration resistance in a similar resistance chain is adjustable either manually or by a densitometer as described above. The invention is applicable to measurement of the interface level of immiscible liquids with different di-electric constants.